1. Field of the Invention
The present invention relates to semiconductor packaging. One example of an application where this present invention may be practiced is on grid array semiconductor packages.
2. Background Art
The following figures which illustrate the various embodiments of the background art and the present invention may incorporate the same or similar elements. Therefore, where the same or similar elements occur throughout the various figures, they will be designated in the same manner.
A Ball Grid Array (BGA) semiconductor package has mechanical and electrical external connections that are made via ball-like solid solder that attach to an array of solder pads both upon the BGA package and upon the medium, hereinafter referred to as the substrate or receiving substrate, to which the BGA package is to be electrically and mechanically attached.
FIG. 1 illustrates the cross section of part of a BGA package and receiving substrate, together with the cross section of a solder ball, as known in the art.
The ball of solid solder 100, that is actually illustrated as a solid hemispherical solder ball, is attached to a solder pad 110, which in turn attached to the lower surface of a BGA package 120. Also shown in this figure is the receiving substrate 130 together with an attached solder pad 140.
The U.S. Pat. No. 5,241,133, MULLEN III et al, entitled `LEADLESS PAD ARRAY CHIP CARRIER` illustrates an example of a typical EGA semiconductor package.
For plastic EGA packages, i.e. EGA packages wherein the integrated circuit is protected by an epoxy resin, the solder balls are typically implemented using a tin-lead (Sn-Pb) alloy having a respective 63-37 weight percentage or a tin-lead-silver (Sn-Pb-Ag) alloy having a respective 62-36-2 weight percentage.
The method of attaching solder balls to both the BGA package and the receiving substrate basically involves a thermal step which causes the solder ball to melt, known in the art as reflow, such attachment being known to those skilled in the art.
For ceramic EGA packages, which are heavier than an equivalent plastic BGA package, the solder balls are typically implemented using a tin-lead (Sn-Pb) alloy having a respective 10-90 weight percentage. The attachment of these types of solder balls is assisted by the addition of a solder cream, that typically comprises a flux and a Sn-Pb-Ag (62-36-2) alloy, that assists in the softening and diffusion of the solder ball. Solder has a melting range from approximately 179.degree. C. (solidus) to 380.degree. C. (liquidus), depending upon the composition of the solder.
U.S. Pat. No. 5,525,834, Fischer et al, entitled `INTEGRATED CIRCUIT PACKAGE` discloses a BGA package that has solder balls with a 90% Pb--10% Sn solder alloy that does not melt and collapse during surface mount assembly. Similarly, the PATENTS ABSTRACT OF JAPAN JP 59 058843 A, vol. 008, no. 155 (E256), 19th Jul. 1984 discloses a 90% Pb--10% Sn solder alloy that is used to form a stand-off, which is covered with a 63% Sn--37% Pb solder alloy, the stand-off acts as a stopper during reflow of the lower melting point 63% Sn--37%. Pb solder. A solder ball having a core having a relatively high melting temperature and a solder coating having a lower melting temperature is disclosed in International Patent Application WO 9701866 A1 970116.
U.S. Pat. No. 5,466,635, Lynch et al, discloses a method of forming a rigid standoff by electroplating. The concept of a rigid standoff is also disclosed in U.S. Pat. No. 5,468,995 Higgins III and a `bump electrode` that is attached by a conductive adhesive layer is disclosed in the European Patent Application 0 724 289 A2.
The soldering of a EGA package, complete with attached solder balls, to a substrate is carried out by the reflow method. The solder balls ensure electrical contact between the BGA package and the substrate and also compensates for defects in the flatness of the surfaces to be electrically and mechanically bonded.
One disadvantage of solder balls is that, mechanically they are not very strong, which is due to their composition. As a result these solder balls quickly foul the machinery employed in their manipulation and placement.
Another disadvantage with solder balls relates to the area and geometry of the pads both on the package and the substrate to which the package is to be bonded. These pads set the tolerances relating to the shape, and thus the height, and spacing of the solder balls that are to mechanically and electrically bond the package and the substrate.
Other disadvantages with solder balls is that they are difficult to clean; difficult to desolder; and difficult to inspect for mechanical and electrical defects, such as dry solder joints etc.